1. Field of the Invention
The present invention relates to an antireflection film. More particularly, the present invention is concerned with an antireflection film comprising silica particles and at least one binder compound, wherein the silica particles are bound together through the at least one binder compound, and wherein the antireflection film has a silica particle content of 30% by weight or more, an arithmetic mean surface roughness (Ra) of not more than 2 nm and a surface silicon atom content of 10 atom % or more. The antireflection film of the present invention not only exhibits excellent antireflection performance, but also has excellent properties with respect to mechanical strength and abrasion resistance. Therefore, the antireflection film of the present invention is very advantageous for coating various optical substrates (such as lenses of eye-glass and display screens).
2. Prior Art
Conventionally, as an antireflection film for coating an optical part, lenses of eye-glasses, a display screen or the like, there are known an antireflection film having a single-layer structure and an antireflection film having a multilayer structure. An antireflection film having a single-layer structure or a double-layer structure has disadvantageously high reflectance. Therefore, it has been considered to be more desirable to use an antireflection film having a laminated structure comprised of three or more different layers having different refractive indices. However, when such an antireflection film comprised of three or more different layers is produced by any of the conventional methods, such as vacuum deposition and dip coating, disadvantages are caused in that the production process is cumbersome and also the productivity is low.
Therefore, studies have been made on antireflection films having a single-layer structure or a double-layer structure, and it has been found that the refractive index of such a single-layer or double-layer antireflection film can be reduced when the antireflection film satisfies the conditions mentioned below. Thus, studies have been made for developing a single-layer or double-layer film which satisfies such conditions. Specifically, it is known that, in the case of an optical part comprising a substrate and, formed thereon, a single-layer film, the minimum value of the reflectance R of the optical part can be expressed by the formula:
(ns−n2)2/(ns+n2)2, wherein ns represents the refractive index of the substrate and n represents the refractive index of the single-layer film, with the proviso that ns>n. When the minimum value of the reflectance R is 0 (i.e., when (ns−n2)2/(ns+n2)2=0), it means that n=ns1/2. Therefore, it has been attempted to reduce the reflectance R by adjusting the refractive index n of the single-layer film to a value which is as close as possible to ns1/2. Further, when it is difficult to adjust the refractive index n of the single-layer film to a value which is close to ns1/2, it has also been attempted to reduce the reflectance R by a method in which a high refraction layer having a high refractive index which is close to n2 is formed between the substrate and the above-mentioned single-layer film having a refractive index n, to thereby obtain a double layer structure.
At present, commercially available products of optical parts having an antireflection film have a minimum reflectance of about 2% in the visible range. However, of these commercially available optical parts, the number of those optical parts having both a minimum reflectance of 2% or less and practically satisfactory properties with respect to mechanical strength and durability, is very small. Therefore, it has been desired to provide an antireflection film which can be easily produced and which has both a minimum reflectance of 2% or less, preferably a minimum reflectance of 1% or less and practically satisfactory properties with respect to mechanical strength and durability.
On the other hand, for the purpose of providing an antireflection film having an increased surface hardness and an anti-dazzling property (achieved by a light scattering property imparted to the surface of the film), it has been attempted to incorporate silica particles into the surface portion of an antireflection film to thereby form minute unevenness (i.e., minute dents and bumps) on the surface of the antireflection film. It is known that, by forming minute dents and bumps on the surface of the antireflection film in such way, there can be obtained, to some extent, the effect of improving the abrasion resistance of the antireflection film. The reason why such effect can be obtained is that the presence of the minute dents and bumps on the surface of the antireflection film can decrease the practical contact area between the antireflection film and an object which placed in contact with the surface of the antireflection film. However, an antireflection film having such a surface structure has posed a problem in that a stress applied to such a roughened surface of the antireflection film is inevitably mainly focused on the minute “peak top” portions (i.e., dents) of the roughened surface and, hence, the surface portion of the antireflection film is partially scraped off and/or the antireflection film is partially crushed in the thicknesswise direction, leading to a partial lowering of thickness of the antireflection film, and this will cause an inadvertent change in the color tone of the antireflection film. When it is tried to solve this problem by a method in which the amount of silica particles used is increased in an attempt to further increase the surface hardness of the antireflection film, the surface roughness of the antireflection film is further increased. This increase in the surface roughness poses problems not only in that there occurs an increase in the frictional resistance when the antireflection film is placed in contact with an object, but also in that there occurs an increase in the coming off of silica particles from the surface of the antireflection film, thus leading to even a lowering of the abrasion resistance (and not an increase in the abrasion resistance). Thus, in the conventional techniques, it is very difficult to control the surface morphology and surface hardness of an antireflection film.
Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 3-150501 and Hei 5-163464 disclose an antireflection film containing silica particles and having minute dents and bumps on the surface thereof. In these patent documents, these antireflection films are described to show good results in the evaluation of the abrasion resistance. However, in these patent documents, the abrasion resistance is evaluated simply by a method in which an antireflection film is subjected to rubbing using a stationery eraser. It is believed that, by such method, the practical performance of an antireflection film cannot be satisfactorily evaluated.
Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 11-292568 and 2000-256040 disclose an antireflection film containing silica particles and having an arithmetic mean surface roughness (Ra) of more than 5 nm. In these patent documents, it is described that, when the antireflection film was subjected to measurement of the abrasion resistance by rubbing the antireflection films with a dry cloth, the water contact angle of the antireflection film changed from 107° to 100°. Such results of the evaluation cannot be considered to show that the antireflection film exhibits a satisfactory practical performance.
Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 4-340902, Hei 7-48117, 2001-188104 and 2001-163906 disclose an antireflection film containing silica particles. However, in the techniques of these patent documents, the use of silica particles is simply intended to improve the optical performance of the antireflection film. In these patent documents, an antireflection film having a low refractive index and a low reflectance is obtained, but the abrasion resistance of the antireflection film is still unsatisfactory.
Unexamined Japanese Patent Application Laid-Open Specification No. 2002-221603 discloses an antireflection film transfer structure comprising a substrate having an arithmetic mean surface roughness (Ra) of from 2 to 150 nm and, laminated thereon, an antireflection layer to be transferred, and also discloses an antireflection film obtained using the antireflection film transfer structure. In this patent document, the purpose of adjusting the arithmetic mean surface roughness (Ra) of the substrate to a value within the range of from 2 to 150 nm is to improve the uniformity of thickness of the antireflection film obtained. This patent document has no description about how the above-mentioned adjustment of the arithmetic mean surface roughness (Ra) contributes to the strength of the antireflection film.
Further, it is known that when an antireflection film is caused to have a surface morphology having only few dents and bumps, i.e., a flat surface, the abrasion resistance of the antireflection film is lowered. For example, International Publication No. WO03/083524 discloses an antireflection film containing silica particles, and has a description that, when the arithmetic mean surface roughness (Ra) of the antireflection film is less than 3 nm, a satisfactory abrasion resistance is not likely to be exhibited. Also, Unexamined Japanese Patent Application Laid-Open Specification No. 2002-79600 discloses an antireflection film containing silica particles, and has a description that, when the arithmetic mean surface roughness (Ra) of the antireflection film is less than 2 nm, a lowering of abrasion resistance occurs.
As described hereinabove, there has not yet been able to be obtained an antireflection film which not only exhibits excellent antireflection performance, but also has satisfactory properties with respect to mechanical strength and abrasion resistance.